1. Field of Invention
The present invention relates to a satellite signal reception device, a timekeeping device, and a satellite signal reception method.
2. Description of Related Art
The Global Positioning System (GPS), which is a system in which satellites (GPS satellites) orbiting the Earth on known orbits transmit signals (satellite signals) carrying superposed time information and orbit information, and a terrestrial receiver (GPS receiver) receives these signals (satellite signals) to determine its own position, is widely known.
The GPS receiver captures signals from a plurality of GPS satellites, acquires satellite information including accurate time information (GPS time information) and orbit information from each of the captured satellites, performs a positioning calculation using the acquired satellite information, and thereby acquires its own location. The time required to acquire the satellite information therefore depends upon the strength of the satellite signal, and the positioning calculation can take a long time to converge.
Japanese Unexamined Patent Appl. Pub. JP-A-2005-106720 teaches a GPS receiver that stores previously acquired satellite orbit information (such as the almanac parameters) in backup memory, and has a warm start mode and a cold start mode. The warm start mode uses the information stored in backup memory to predict the GPS satellites that can be currently acquired, and shortens the time required from startup to determining the position by attempting to capture those satellites. In the cold start mode the GPS receiver sequentially attempts to capture all of the GPS satellites in the constellation.
The power consumption required from startup to positioning is preferably as little as possible in applications where low power consumption is needed.
When there are few opportunities to acquire the positioning information and the interval between such opportunities is long (such as in a timepiece that adjusts the time difference based on the positioning information), however, the valid period (several weeks in the GPS system) of the previously acquired satellite orbit information (such as the almanac parameters) is often passed. This results in the GPS receiver always starting up in the cold start mode with almost no chance to start in the warm start mode.